In situ decarbonylation of N,N-dimethylformamide to form dimethylammonium cations in the hybrid framework compound {[(CH3)2NH2]2[Zn{O3PC6H2(OH)2PO3}]}n

In the title hybrid organic–inorganic compound, the (CH3)2NH2 + cations interact with the zinc–phosphonate framework via N—H⋯O hydrogen bonds. The (CH3)2NH2 + cations were formed by the in situ decarbonylation of the N,N-dimethylformamide (DMF) solvent.


Chemical context
Studies on the structural chemistry of metal phosphonates developed as a result of the versatility of the phosphonate ligands (Zubieta et al., 2011;Mao, 2007;Clearfield, 1996Clearfield, , 1998Clearfield, , 2002. A slight modification of the organic residues of the phosphonic acids (R-PO 3 H 2 , where R = organic residue) can lead to rich structural diversity. In general, phosphonates tend to assume various coordination modes as a result of the three coordinating oxygen atoms of the central phosphorus units. As a consequence, most metal phosphonates form a low-dimensional and dense layered structure (Deria et al., 2015;Gagnon et al., 2012). Nevertheless, a large number of isolated metal phosphonates have shown various potential applications in ion-exchange, ionic conductivity, gas storage, catalysis, and as small molecule sensors and magnetic interactions (Adelani & Albrecht-Schmitt, 2010;Ramaswamy et al., 2015;Deria et al., 2015;Kirumakki et al., 2008;Brousseau et al., 1997;. The majority of metal-organic frameworks (MOFs) are designed with carboxylate-and nitrogen-containing heterocyclic ligands, while phosphonate-based MOFs are less well studied. One possible explanation may have to do with the predisposition of phosphonates to precipitate rapidly into less ordered insoluble phases. However, carboxylate-based MOFs are less stable in air and water, and this poses a significant problem if they are to be used in industrial applications. Metal carboxylate MOFs are subject to hydrolysis and are quite soluble in acidic solutions. On the contrary, phosphonates manifest stronger interactions with oxophilic metal ions than carboxylates and are not subject to hydrolysis (Deria et al., 2015;Gagnon et al., 2012).

Structural commentary
The structure of (I) crystallizes in the monoclinic space group P2 1 /n. The asymmetric unit contains one Zn 2+ cation, a C 6 H 4 P 2 O 8 4À hydroxyphosphonate tetra-anion and two (CH 3 ) 2 NH 2 + cations (Fig. 1). The extended structure is constructed from tetrahedral ZnO 4 units with the O atoms arising from four rigid phenyl spacers into a three-dimensional framework (Fig. 2). Two of the oxygen atoms of each PO 3 2À moiety are involved in coordination to the Zn 2+ ion and the others (O2 and O6) are not. The Zn-O bond distances range from 1.9055 (11) to 1.9671 (11) Å and the hydroxyphosphonate ligand is present in (I) with P-O bonds that range from 1.5129 (11) to 1.5337 (11) Å in length. The latter bond lengths are within the expected range for deprotonated P-O bonds (Liang & Shimizu, 2007).
The structure of (I) is similar to that of {[Zn(DHBP)](DMF) 2 } (Liang & Shimizu, 2007;CCDC refcode JIVFUQ) in that the zinc-phosphonate framework comprises one-dimensional channels occupied by guest species, but with the significant difference that the guest species in JIVFUQ are neutral DMF molecules and the phosphonate groups are singly, rather than doubly deprotonated to form C 6 H 6 P 2 O 8 2À dianions. The channels reported here are smaller than those in JIVFUQ and measure approximately 12.9 Â 7.1 Å between phenyl groups and 9.9 Å between Zn centers. The (CH 3 ) 2 NH 2 + cations in (I) have been formed by the in situ decarbonylation of the DMF solvent. It is known that N,Ndimethylformamide can undergo loss of CO to form dimethylamine in the presence of a metal catalyst or through slow decomposition at elevated temperature around 427 K (Hulushe et al., 2016;Siddiqui et al., 2012;Chen et al., 2007;Karpova et al., 2004). In the previous reports, the nitrate salts of Mg 2+ /Pb 2+ /Ho 3+ and chloride salts of Nd 3+ /Zr 4+ were suggested to act as a metal catalyst in the decarbonylation of the DMF solvent. View down [100] of the three-dimensional framework structure of (I) with the ZnO 4 and PO 3 C moieties shown as polyhedra. Color key: ZnO 4 groups = cyan, PO 3 C groups = magenta, oxygen = red, carbon = black, hydrogen = white. The (CH 3 ) 2 NH 2 + cations are omitted for clarity.

Supramolecular features
The C6-O8H and C3-O7H groups appended on the phenyl ring of the ligand form intramolecular O-HÁ Á ÁO hydrogen bonds with the adjacent RPO 3 2À moieties ( Figs. 1 and 3). Within the channels, the (CH 3 ) 2 NH 2 + cations are linked by N-HÁ Á ÁO hydrogen bonds to the RPO 3 2À groups of the framework (Table 1). Some short C-HÁ Á ÁO contacts (Table 1) may help to consolidate the structure.

Synthesis and crystallization
The title compound was synthesized by placing Zn(NO 3 ) 2 Á6H 2 O (29.7 mg, 0.1 mmol) and 2,5-dihydroxy-1,4benzenediphosphonic acid (27.0 mg, 0.1 mmol) into a 125 ml PTFE-lined Parr reaction vessel along with DMF/H 2 O/ ethanol (2.0/0.5/0.5 ml, respectively). The vessel was heated in a programmable furnace at 353 K for 3 d, and then the autoclave was cooled to 296 K at an average rate of 274 K h À1 . The mother liquor was decanted from the products and then placed in a petri dish. The solid products were washed with distilled water, dispersed with ethanol and allowed to dry in air. Colorless tablets of the title compound were isolated and studied for single-crystal X-ray diffraction.

Figure 3
Ball-and-stick representation of the structure of (I) viewed along the

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.